The present invention is directed toward a raster output scan (ROS) printer and more particularly to a digital printer in which the scanning beam is produced by directing a variable wavelength output of a laser diode through a dispersive element.
Modern raster scan printers typically consist of a laser light source, a modulator, a beam deflector, an optical system of lenses and mirrors, a xerographic marking engine and the electronics to control the printer operation. Of critical importance to these printers is the deflector which is used to create the raster output scan at the surface of the imaging member, usually a photoreceptor belt or drum. Several deflection techniques are known in the art. An electrooptical system is disclosed in U.S. Pat. No. 4,450,459, where a laser output is directed across the full width of an electro-optic element. Individual electrodes are addressed to produce a diffraction pattern resulting in a beam of light sweeping across an image recording medium to produce a raster scan output. This type of prior art system has been found to be unsatisfactory for high resolution, high speed printing needs. Polygon scanning beam deflectors have come to be the preferred beam deflection element in most commercial ROS printers, due to its design simplicity and reliability at high scanning resolution and speed. U.S. Pat. No. 4,349,847 discloses a conventional polygon ROS type of printing system.
A polygon scanner has certain limitations and disadvantages. The polygon is typically mounted on a shaft driven at a high rotational speed by a motor. The scanner, especially at high speeds, is subject to motion (wobble) error which must be corrected for in the post-polygon optics. The bearing on which the polygon is mounted to the shaft is subjectto wear and this can result in a misalignment of the scan line at the image surface. It would be desirable to utilize a nonmechanical passive beam deflector if the high performance characteristics associated with the polygon scanner could be retained. The present invention is directed to such a ROS scanner which utilizes the wavelength varying output of a laser source in combination with a stationary light deflector, which, in a preferred embodiment, is a grating. The beam is diffracted from the grating as a function of the wavelength of the incident light. The diffraction angle varies over the wavelength range to produce an output beam which is focused at the image plane as a raster scan line. This concept of selectively varying the wavelength of laser output and directing the output to a dispersive element for scanning purposes is generally described in U.S. Pat. No. 4,250,465. There is also significant literature applied to different techniques of accomplishing wavelength scanning. For example, Filnski and Skettrup in an article entitled "Fast Dispersive Beam Deflectors and Modulators" (IEEEJ. Quantum Electronics, QE Vol. 18, No. 7, pg., 1059-1062 July, (1982) proposed electro-optic tuning of the output of a broadband laser and directing the output to a two dimensional scanner (prism). Examples of a tunable laser diode having wide wavelength scanning range are disclosed in an article by Mittelstein, et al. Applied Physics Letters 54, Mar. 20, 1988, 1092 and by Hall, et al. in an article entitled "Broadband Long Wavelength Operation" Appl. Phys. Lett., Vol. 58 No. 8 8, Aug. 21, 1989, pp 752-754.
While the prior art literature has provided suggestions for using the scanned output of the dispersive element in various applications (U.S. Pat. No. 4,250,465, col. 1, lines 11-16), there has been no exploration of the type of system necessary to enable a ROS scanner capable of forming high resolution scan lines at the surface of a photoreceptor belt or drum, e.g. high resolution high scan printing.
To enable a high resolution digital ROS printer using the variable wavelength tuning technique, the laser source must provide a tunable range of between 50 to 100 nm of the laser wavelength and the dispersive element must be able to provide up to 1000 resolvable spots per inch or 10,000/scan lines at the image surface. Further optics may be necessary for correction of any nonlinearity associated with the diffracted beam and compensation for varying spot size due to changing the effective aperture and length. According to the present invention, a high resolution ROS system is accomplished by utilizing a scannable laser within an external resonator cavity. The laser, a diode laser in the preferred embodiment, is made to operate in a single longitudinal mode of the external resonator from among the plurality of longitudinal modes that can exist for the laser of a given length and spectral bandwidth of the gain medium. The laser wavelength is scanned through the longitudinal modes of the laser. This is referred to as a "digital scanner" because only discrete wavelengths, hence spot positions in the image plane, are allowed by the fixed laser cavity length. Scanning the laser wavelength steps sequentially through the longitudinal modes. An expanded output beam is diffracted along a scan path by a high resolution grating and focused as a scan line on the image plane by a scan lens designed to correct for the distortional effects introduced by the grating. More particularly, the invention is directed to a digital scanning laser printer for forming high resolution scan lines at a photosensitive image medium comprising: a broad-band wavelength tunable laser source operational in a plurality of longitudinal modes to provide an output beam of radiation whose wavelength varies incrementally over the tunable range, a laser cavity within which said laser source is positioned, said cavity having a length L given by the expression EQU L=.lambda..sup.2 /.sub.2n(FSR)
where FSR is the scan line pixel spacing, n is an integer and .lambda. is the laser source operating wavelength, a diffraction grating in the path of said output radiation beam, said grating adapted to diffract said incident radiation as a function of its wavelength to produce an output beam which scans across a diffraction angle, and a scan lens positioned between the photosensitive imaging plane and the grating, said scan lens adapted to focus the scanned diffracted beam unto a scan line of the photosensitiveimage medium while correcting for distortion and nonlinearity caused by the grating.